The engine control module (ECM) uses the generator field duty cycle signal circuit, or F-terminal circuit, to monitor the duty cycle of the generator. The generator field duty cycle signal circuit connects to high side of the field windings in the generator. A pulse width modulated (PWM) high side driver in the voltage regulator turns the field windings ON and OFF. The ECM uses the signal to determine the generator load on the engine. This allows the ECM to adjust the idle speed to compensate for high electrical loads. The ECM monitors the status of the generator field duty cycle signal circuit. When the ignition switch is in the RUN position and the engine is OFF, the ECM should detect a duty cycle near 0 percent. However, when the engine is running, the duty cycle should be between 5-99 percent.

The ECM detects that the field duty cycle signal is less than 5 percent for 30 seconds.

The charge indicator turns ON.

Schematic Reference

Starting and Charging Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Charging System Description and Operation

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

1. Ignition ON, engine OFF, observe the scan tool ECM GEN-F Terminal Signal parameter. The reading should be less than 5%.
2. Engine running, observe the scan tool ECM GEN-F Terminal Signal parameter. The reading should be greater than 5%.

1. Verify that a test lamp illuminates between the B+ circuit ring terminal 1 X2 and ground.

⇒	If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance.

2. Ignition OFF, disconnect the X1 harness connector at the generator.
3. Ignition ON, verify the scan tool ECM GEN-F Terminal Signal parameter is less than 5%.

⇒	If greater than the specified range, test the appropriate signal circuit terminal listed below for a short to voltage. If the circuit tests normal, replace the ECM.

•	Terminal 2 X1 (L61/LE5)

•	Terminal C X1 (LNF)

4. Install a 3A fused jumper wire between the appropriate signal circuit terminal listed below and B+. Verify the scan tool ECM GEN-F Terminal Signal parameter is greater than 95%.

•	Terminal 2 X1 (L61/LE5)

•	Terminal C X1 (LNF)

⇒	If less than the specified range, test the circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the ECM.

5. If all circuits test normal, test or replace the generator.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.